LT1943EFE Linear Technology, LT1943EFE Datasheet - Page 14

• Manufacturer Part Number: LT1943EFE
• Description: IC REG SW QUAD TFT LCD 28-TSSOP
• Manufacturer: Linear Technology

Specifications of LT1943EFE

• Applications: Converter, TFT, LCD
• Voltage - Input: 4.5 ~ 22 V
• Number Of Outputs: 4
• Voltage - Output: 1.25 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP Exposed Pad, 28-eTSSOP, 28-HTSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

LT1943
OPERATIO
Table 3. Schottky Diodes
Part Number
On Semiconductor
MBRM120E
MBRM140
Diodes Inc.
B120
B130
B220
B230
B240
International Rectifier
10BQ030
20BQ030
Boost Pin Considerations
The minimum operating voltage of an LT1943 application
is limited by the undervoltage lockout ~4V and by the
maximum duty cycle. The boost circuit also limits the
minimum input voltage for proper start-up. If the input
voltage ramps slowly, or the LT1943 turns on when the
output is already in regulation, the boost capacitor may not
be fully charged. Because the boost capacitor charges
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on input
and output voltages. The Typical Performance Character-
istics section shows a plot of the minimum load current to
start as a function of input voltage for a 3.3V output. The
minimum load current generally goes to zero once the
circuit has started. Even without an output load current, in
many cases the discharged output capacitor will present
a load to the switcher that will allow it to start.
INVERTER/STEP-UP CONSIDERATIONS
Regulating Positive Output Voltages
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistors
according to:
14
R3 = R4(V
OUT
V
/1.25 – 1)
R
U
20
40
20
30
20
30
40
30
30
(V)
I
AVE
1
1
1
1
2
2
2
1
2
(A) V
F
at 1A (mV)
530
550
500
500
420
V
F
at 2A (mV)
595
500
500
500
470
470
R4 should be 10kΩ or less to avoid bias current errors. If
switcher 4 is used to generate a positive output voltage,
NFB4 should be tied to FB4.
Regulating Negative Output Voltages
The LT1943 contains an inverting op amp with its non-
inverting terminal tied to ground and its output connected
to the FB4 pin. Use this op amp to generate a voltage at FB4
that is proportional to V
ing to:
Use 10k for R5. Tie 10pF in parallel with R5.
Duty Cycle Range
The maximum duty cycle (DC) of the LT1943 switching
regulator is 85% for SW2, and 83% for SW3 and SW4. The
duty cycle for a given application using the step-up or
charge pump topology is:
The duty cycle for a given application using the inverter or
SEPIC topology is:
R
DC
6
=
=
R
|
V
5
1 245
OUT
|
• |
.
V
OUT
V
0
| –
UT
V
V
|
IN
|
C1
10pF
V
OUT
OUT4
R3
R4
–V
OUT
R6
R5
10k
. Choose the resistors accord-
NFB4
1943 A2
FB4
NFB4
1943 A1
FB4
1943fa